As tetraethyl lead is phased out, oxygenates have become more important in the petroleum refining industry as a source of gasoline octane boosters. The most common oxygenates for this purpose are the dialkyl ethers, and especially those in the C.sub.5 to C.sub.7 range. One such dialkyl ether that is generating much interest is diisopropyl ether (DIPE). DIPE is in the boiling range of gasoline, has a high blending octane number, and one reactant generally used in the formation of DIPE, propylene, is a by-product commonly available in refineries. The preparation of DIPE from propylene chemically proceeds by two sequential reactions, where propylene is first hydrated to isopropyl alcohol (IPA) (1) followed by reaction of the alcohol with the olefin (2) or bimolecular dehydration reaction of the alcohol (3) (Williamson synthesis) according to the equations, ##STR1##
Hydration reactions such as reaction (1) are generally more difficult to perform than the etherification reaction (2) and the dehydration reaction (3). For example, severe conditions such as pressures as high as 500 to 1500 psig are required for hydration since in their normal states propylene is a gas and water is a liquid and the solubility of propylene in water is small. Therefore, the production rate of isopropyl alcohol by reaction (1) limits the rate of the overall sequence. Maximum overall production rate is achieved when the etherification is substantially accomplished by reaction (2) since reaction (2) only consumes one mole of IPA to produce one mole of DIPE, while in contrast, reaction (3) consumes two moles of IPA to produce one mole of DIPE. Reaction (2) is also preferred over reaction (3) on the basis of cost. The hydration reaction to produce IPA is costly as well as difficult, so it is advantageous to minimize IPA consumption in the reaction to produce DIPE. Propylene, on the other hand, is relatively low in cost since it is typically available as a by-product from other processes in a refinery.
The olefin hydration and etherification or alcohol dehydration reactions may be carried out in a single stage, or in two stages. As discussed above, the hydration reaction conditions are very different from the etherification reaction conditions, and an advantage of the two-stage design is that each stage may be independently optimized for a particular reaction. For example, U.S. No. 5,144,086 disclosed a DIPE production process using two separate reactors, the first dedicated to forming IPA by olefin hydration and the second dedicated to forming DIPE by dehydration of IPA. In this process, propylene and water are introduced to a first reactor containing an acidic hydration catalyst to produce an IPA-containing effluent that is substantially free of DIPE. After separating the effluent stream to remove the propylene and propane, the IPA-containing stream is passed to a second reactor containing a zeolite catalyst to produce an effluent that contains DIPE, water and propylene. The effluent from the second reactor is then fractionated to produce the DIPE product. It is important to note that because any propylene is removed prior to the second reactor, this process produces DIPE only by reaction (3) and not at all by reaction (2) therefore requiring two moles of IPA to be made in the first reactor for each mole of DIPE produced.
Similarly, in the DIPE production process disclosed in U.S. No. 4,935,552, propylene, water, and IPA are introduced to a first reactor containing a hydration/etherification catalyst to form an effluent which consists of IPA, DIPE, and unconsumed reactants. The effluent is then flashed to remove any propylene and extracted with water to transfer the IPA to the aqueous phase. A portion of the resulting hydrocarbon phase containing high purity DIPE is recycled to the first reactor in order to control the temperature. The aqueous IPA phase is passed to a second reactor, which is a catalytic distillation unit, where dehydration of the IPA to form DIPE takes place. It is important to note that in this second reactor, DIPE is made exclusively by reaction (3) since the propylene has all been removed. Consequently, the first reactor must provide two moles of IPA for every mole of DIPE produced.
Applicants' invention for the production of DIPE addresses the prior art drawback of high IPA consumption. Applicants' invention reduces production costs through more efficient use of the IPA produced in the hydration zone by optimizing the etherification zone to produce DIPE according to reaction (2) instead of exclusively by reaction (3). Additionally, applicants' invention increases DIPE product yield beyond thermodynamic equilibrium limitations and consolidates separation equipment through performing the etherification by catalytic distillation.